Seawater adaptation by out-of-season Atlantic salmon (Salmo salar L.) smolts at different temperatures

Atlantic salmon smolts (Salmo salar L.) were transferred to full-strength seawater for 0 (initial control group), 0.5, 1, 2, 4, 8, 14, 30, 42 and 60 days at four different temperatures (4.6, 9.1, 14.4 and 18.9 degrees C). Water temperature in each tank was adjusted during the last 5 days in freshwater (ambient 8 degrees C) to gradually establish test conditions of 4.6, 9.1, 14.4 and 18.9 degrees C 24 h prior to transfer, Thereafter, the water in all tanks were changed from freshwater to seawater of identical temperature, and full salinity (33 parts per thousand) was reached within 60 min. Physiological adaptation was measured as changes in plasma growth hormone levels, gill Na+,K+-ATPase activity, plasma chloride levels and muscle water content. The fatty acid composition of gill tissues was determined after 30 days in seawater. Ion and water balance following seawater transfer were significantly affected by temperature. Exposure to high temperatures (18.9 degrees C) resulted in a rapid increase in plasma chloride levels and a rise in tissue dehydration within 24 h, whereas low temperatures (4.6 degrees C) resulted in a delayed osmotic disturbance and a prolonged period of osmotic stress. Least osmoregulatory disturbance was observed at 9.1 degrees C. Gill Na+,K+-ATPase activity did not increase after seawater exposure at 4.6 and 9.1 degrees C, whereas a gradual increase was observed with increasing temperatures at 14.3 and 18.9 degrees C during the first 48 h in seawater. Plasma GH increased in all groups during the first 24 h of seawater exposure. GH levels decreased during long-term adaptation in the 4.6, 9.1 and 14.4 degrees C groups, whereas a significant increase was observed in the 18.9 degrees C group. Growth increased with increasing temperature between 4.6 and 14.4 degrees C, but decreased significantly between 14.4 and 18.9 degrees C demonstrating that 18.9 degrees C is above optimum for growth and development in seawater. (C) 2000 Elsevier Science B.V. All rights reserved.